Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
OPERATING ACTIVITIES
Net income	$ 22,361	$ 17,803	$ 39,679	$ 31,182
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			45,297	50,743
Deferred income taxes			4,421	4,367
Stock-based compensation			1,279	319
Gain on disposal of property and equipment	(7,542)	(5,386)	(11,383)	(8,254)
Changes in certain working capital items:
Trade receivables			(4,688)	5,810
Prepaid expenses and other current assets			(1,148)	(2,087)
Accounts payable, accrued liabilities, and accrued expenses			(2,727)	(11,073)
Accrued income taxes			946	(1,286)
Net cash provided by operating activities			71,676	69,721
INVESTING ACTIVITIES
Proceeds from sale of property and equipment			38,752	64,359
Purchases of property and equipment, net of trades			(66,483)	(86,936)
Change in other assets			7	460
Net cash used in investing activities			(27,724)	(22,117)
FINANCING ACTIVITIES
Payments of cash dividends			(1,640)	(3,311)
Shares withheld for employee taxes related to stock-based compensation			(321)	(124)
Repurchases of common stock			0	(20,922)
Net cash used in financing activities			(1,961)	(24,357)
Net increase in cash, cash equivalents and restricted cash			41,991	23,247
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period			182,938	106,098
Ending Period	224,929	129,345	224,929	129,345
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds			7,202	1,598
Noncash investing and financing activities:
Purchased property and equipment in accounts payable			13,118	511
Sold revenue equipment in other current assets			2,448	5,600
Dividends Payable	1,640	0	1,640	0
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents, at Carrying Value	205,642	106,402	205,642	106,402
Restricted Cash and Investments, Current	2,404	2,819	2,404	2,819
Restricted Cash and Cash Equivalents, Noncurrent	16,883	20,124	16,883	20,124
Ending Period	$ 224,929	$ 129,345	$ 224,929	$ 129,345